Well, let me try to clear this point. First, I'm sure you know "seu, sua, seus, suas" mean "your, yours". I suppose you're asking about another phenomenom, when those pronouns can be replaced by "dele, dela, deles, delas." As a rule of thumb, use "seu, sua..." in more formal writing, avoiding "dele, dela...", which are more appropriate for casual Portuguese. However, there's a problem with that rule. You must notice if you're sentence does not look ambiguous. Let's suppose you're married and, as we were talking, I used the following structure: - Então, quando eu percebi, a Lúcia tinha saído com seu marido... (Then, when I realized, Lúcia had gone out with her/your husband...) The question which arises in constructions like that is who is the referent of "seu". The phrase is ambiguous because one cannot know wether Lúcia has gone out with her husband or with the listener's husband (in this case, you). That's the only point you must be concerned at. A sentence like that can even cause a divorce to happen! :-) Hopefully I have answered your question. For further information, feel free to contact me. Yours, Mateus.